In a catchment area study of 101 first inceptions of schizophrenia, mania and atypical psychoses, women were significantly more likely to have atypical psychosis and men were more likely to have definite schizophrenia. Negative symptoms such as affective flattening and poverty of speech were already present in many cases, and were significantly increased in patients with definite schizophrenia (geometric mean 5.6) compared with those with atypical psychosis (geometric mean 3.2) and mania (geometric mean 1.5). Negative symptoms were also twice as severe in men (geometric mean 5.5) than women (geometric mean 2.6). There was a significant increase in negative symptom severity with longer illness and greater depression, but the diagnosis and the sex effects were not caused by these factors. We suggest that our findings are further support for the hypothesis that men have a greater biological vulnerability to negative symptoms and consequent social disability in the face of psychosis, particularly a schizophrenic psychosis, and that this may be one explanation for the apparently greater risk of definite schizophrenia and its poorer prognosis in men.